About Color QuickDraw and System 7.0
This section describes extensions to the color facilities of
Color QuickDraw. It discusses other new features, notably luminance
mapping techniques, new routines for copying pixel maps, extensions to the
version 2 picture format, and routines by which existing applications can
signal QuickDraw that a data structure has been modified directly.
Graphics Devices Manager sections, and with the description of the Slot
Manager in Designing Cards and Drivers for the Macintosh Family, second
edition.
If you use offscreen graphics to prepare images before copying them to the
routines that considerably reduce the complexity of that task.
Color QuickDraw in system 7.0 or later supports images that use direct,
as well as indexed, specification of pixels.
Although an application specifies a color in terms of RGB space, the actual
is an index value, which is used as input to the color look-up table (CLUT). An
8-bit index can specify 256 different entries (28 = 256). On CLUT devices,
the relationship between the index value and the color generated depends on the
colors and their current locations in the table.
At the cost of larger RAM requirements and, in some situations, slower
performance, direct pixel specification eliminates the need for color table
look-ups and inverse tables, and it lets your application directly specify over
16 million colors. (For a comparison of the differences between indexed and
• new facilities for copying pixel map records
• support for a true gray-scale display, providing better image fidelity
on gray-scale devices
• a routine that converts a bitmap record to a region, allowing you, in
effect, to use region-manipulating routines on bitmap or 1-bit pixel
map records
• a routine that creates pictures with variable dots-per-inch (dpi)
resolution
application directly modifies a device's color table, pixel pattern,
grafPort, or graphics device record (direct modification is still
discouraged)
The rest of this section
• describes the changes to the pixel map record that support direct
pixel values, and presents examples of converting between various
pixel depths
data structures directly, rather than using the routines provided for
a direct change has taken place
• describes new techniques for copying and displaying pixel maps
• describes new routines for converting a bitmap to a region and
determining when drawing to a port has completed
• presents extensions to the version 2 picture format that support
direct pixels and store font and line justification information
Direct Pixels
Color QuickDraw now supports pixel maps with direct pixel specification as
well as the indexed method supported by the original release of Color
many applications need make no changes. If your application specifies RGB
colors, the system determines the best matching colors for indexed devices and
passes your RGB colors to direct devices.
only these two color-specification data structures:
• the pixel map record that describes an image
• the version 2 picture format in which images and graphics drawing
operations are stored
The new version 2 picture opcodes are described in the section en titled,
Extensions to the Version 2 Picture Format; extensions to the pixel
map record are described next.
Pixel Map Record Extensions
Color QuickDraw supports two new pixel formats corresponding to 16-bit
and 32-bit pixel depths. In both cases, the pixel's displayed color is directly
specified by the pixel value; the pixel value is not an index into a color table.
that introduced with the Macintosh II computer, but six pixel map fields can
have new values. This section describes only those fields.
Direct Pixel Values
This section presents some examples of direct pixel values and the results of
converting between direct and indexed pixels. The Figure below shows a 32-bit
direct pixel value in which the pixel and component fields have been set up as
pixelType = 16; RGBDirect
pixelSize = 32; must be a power of 2
cmpCount = 3; red, green, and blue values
cmpSize = 8; 8 bits for each component
A 32-bit direct pixel
In this example, the pixel value (hexadecimal) is $00178609, which
deconstructs into component values of $17 red, $86 green, and $09 blue,
resulting in a medium green. The Figure below approximates the same color as
the one in the Figure above using a 16-bit pixel specified as follows:
pixelType = 16; RGBDirect
pixelSize = 16; must be a power of 2
cmpCount = 3; red, green, and blue values
cmpSize = 5; 5 bits for each component
A 16-bit direct pixel
Here the pixel value is $0A01, with component values of $02, $10, and $01
for red, green, and blue.
When converting a 32-bit pixel value to 16 bits, the 3 least significant bits
are dropped for each component. When converting a 16-bit pixel value to 32
bits, the most significant 3 bits of each component are replicated and added to
constitute the least significant 3 bits of each 8-bit component, as illustrated
in the Figure below.
Converting a 16-bit direct pixel to a 32-bit direct pixel
In this way, white remains white, black remains black, and other values are
spread evenly.
RGB color to 32-bit, 16-bit, and 8-bit values (the latter indexed) and then
the reverse process of converting those values back to 48 bits.
A 32-bit direct pixel uses the most significant 8 bits of each component, with
8 unused bits in the high byte, as shown in the Figure below.
Converting a 48-bit RGB color to a 32-bit direct pixel
A 16-bit direct pixel uses the most significant 5 bits of each component and
has an unused high bit, as shown in the Figure below .
Converting a 48-bit RGB color to a 16-bit direct pixel
To obtain an 8-bit indexed pixel value from a 48-bit RGB color, the Color
Manager determines the closest RGB value in the CLUT; its index value is stored
in the 8-bit pixel. In the standard 8-bit color table, the 'CLUT' resource
whose ID is 8, the nearest value to the original RGB value of the Figure above is
in table entry 161, as shown in the Figure below. Note that with indexed
pixels, the pixel value has no direct relation to the original RGB value.
Converting a 48-bit RGB color to an 8-bit indexed pixel
48-bit value by dropping the unused high byte and replicating each 8-bit
component. Note that the resulting 48-bit value differs (in the least
significant 8 bits of each component) from the original value.
Converting a 32-bit pixel to a 48-bit RGB color
Color QuickDraw expands a 16-bit pixel to a 48-bit RGB color by
dropping the unused high bit and inserting three copies of each 5-bit
component and a copy of the most significant bit into each 16-bit component of
the destination value. Note that the result differs (in the least significant 11
bits of each component) from the original value.
Converting a 16-bit pixel to a 48-bit RGB color
Color QuickDraw expands an 8-bit indexed pixel to a 48-bit RGB color by
taking the 48-bit value pointed to in the CLUT, as shown in the Figure below.
The difference between this value and the original 48-bit value varies,
depending on the CLUT values.
Converting an 8-bit indexed pixel to a 48-bit RGB color